1. Field of the Invention
The present invention generally relates to an apparatus useful in downhole logging tools used in the oil and gas industry. The present invention is particularly directed to a tool including improved articulated transducer pad assemblies for providing improved transducer contact with the wall of a borehole or the inside wall of casing cemented in the borehole.
2. Description of the Background
Many measurements using acoustic transducers are made in the oil and gas industry in open and cased wells. Logging tools, in particular sondes from which these acoustic transducers are disposed, are passed through a borehole or a completed well to provide valuable information concerning the completed well and the formation through which a borehole has been drilled.
One of the significant uses for such acoustic logging tools is in determining the integrity of the cementing operation. In the oil and gas industry, wells are completed by setting a string of pipe or casing in a borehole and filling the annulus between the pipe or casing and the borehole with cement. This operation separates the various formation zones, and particularly separates the productive oil and gas bearing formations from non-productive formations such as water bearing formations. Once separated by the cementing operation, only the desired oil and gas bearing formations are perforated for production. Failure or incompleteness of the cement bond likely will result in incomplete separation between the various formations. Migration of fluids under pressure through voids or cracks appearing in the cement between the casing and borehole will cause contamination of the fluids of one zone with the fluids of another zone. This contamination is particularly undesirable where fluids from water bearing strata migrate into fluids in a producing zone. This migration typically results in decreased production of hydrocarbon fluids, increased production of non-desirable fluids and contamination of equipment. These results often cause serious financial loss and may even cause a well to become non-commercial.
Accordingly, the desirability of accurately determining the quality of the cement bond between a casing and a wall of a borehole has long been a goal of those associated with the oil and gas industry. A "good bond" produces the desired separation of zones and is achieved with good adhesion, although microfissures or a microannulus may be present.
Many prior systems have been developed for investigating the bond quality between a casing and a borehole wall. Prior systems for inspecting the cement bond of wells in the oil and gas industry have included conventional acoustic cement bond systems employing acoustical transducers in a variety of devices and methods. One such system is described in U.S. Pat. No. 4,255,798 to Havira. Further, the Havira patent lists and describes in columns 2-7 a plurality of patents and articles describing in detail many prior systems for evaluation of cement bond quality using acoustic energy. The Havira patent and its extensive description of these prior systems are incorporated herein by reference.
Each of the prior systems employs an acoustic logging tool having disposed thereon a plurality of acoustic transducers. These transducers are deployed about or from the logging tool by many means both simple and complex. However, there has been a long felt but unfulfilled need within the oil and gas industry for an apparatus for deploying a plurality of acoustic transducers from a central tool to positions about the inside of a casing cemented in a borehole and for providing means whereby each transducer is independently biased about a plurality of axes so that each transducer exhibits an improved contact with the inside wall of the casing.